In the production of paper from wood fibers, the wood fibers must be freed from the raw wood. In one widely used method, this is accomplished by cooking the wood fibers in a solution until the material holding the fibers together, lignin, is dissolved. In order to achieve rapid and uniform digestion by the cooking liquor, the wood, after it has been debarked, is passed through a chipper, which reduces the raw wood to chips on the order of one inch to four inches long. The chipper tends to produce a large percentage of over-thick chips which, after separation by a screen, must normally be reprocessed through a slicer to reduce them to the desired thickness. This reprocessing through a slicer has the undesirable effect of creating excessive fines and pins. The production of fines and pins reduces the overall yield of high quality fibers from a given amount of raw wood. Because the cost of the raw wood is a major contributor to the cost of paper produced, reslicing the oversized chips incurs a considerable cost.
A long studied but only recently commercialized alternative to reslicing over-thick wood chips is a process known as "destructuring" the chips. The chips are fed through opposed rollers which compress the chips as they pass through the nip of the rollers. The compression of the chip results in longitudinal fractures along the grain and fractures across the grain of the wood. The cracks induced in the chips allow the cooking liquor to penetrate the interior of the chip, thus effectively reducing the chip's thickness. However, the cross-grain cracking results in fibers that produce a paper with lower strength characteristics. For this reason, it is desirable to induce only longitudinal cracks along the grain of the chips.
My earlier U.S. Pat. No. 4,953,795 discloses an apparatus employing aggressively contoured roll surfaces consisting of a matrix of pyramid projections on the roll surfaces. My earlier patent teaches rolls which destructure the wood chips by cracking them preferentially in the direction of the grain.
The use, as disclosed in my earlier patent, of aggressively contoured roll surfaces having a matrix of outwardly extending discrete projections has proven critical to the practical utilization of the chip destructuring process for the preparation of wood chips. Although apparatus to remove tramp metal and other noncompressible articles from the wood chip flow will always be employed, on rare occasions such materials may find their way between the destructuring rolls. As the aggressively contoured roll surfaces are expensive to fabricate it would be desirable to provide rolls which have some endurance when subjected to unexpectedly hard objects. Furthermore, the surfaces, if damaged, should be replaceable with a minimum of down time.